In the recording method by means of an ink jet printer which is one of the typical methods among various color recording methods, ink droplets are generated and adhered onto various record-receiving materials (such as paper, film and cloth) to perform recording. This method has been rapidly prevailing lately and is expected to continue growing remarkably in the future because of such features as quietness with less noise generation due to no direct contact of a recording head with a record-receiving material and as easiness in downsizing and speedup. Conventionally, as an ink for fountain pens, felt-tip pens and the like and an ink for inkjet recording, water-based inks where a water-soluble dye is dissolved in an aqueous medium have been used, and in these water-based inks, a water-soluble organic solvent is generally added to prevent ink from clogging at a pen tip or an inkjet nozzle. Therefore, these inks are required to provide recorded images with sufficient density, not to clog at a pen tip or a nozzle, to dry quickly on record-receiving materials, to bleed less, to have excellent storage stability and so on. In addition, a water-soluble dye to be used is required to have high solubility especially in water and in a water-soluble organic solvent to be added to the ink. Further, formed images are required to have image fastnesses such as water fastness, light fastness, ozone gas fastness and moisture fastness.
Among these, the ozone gas fastness means durability against the phenomenon that ozone gas and the like having oxidizing effect, which exist in the air, react with a dye in recording paper to cause discoloration or fading of printed images. Besides ozone gas, oxidizing gases having this type of effect include NOx, SOx and the like, and ozone gas is, among these oxidizing gases, regarded as a main causative substance to further promote the phenomenon of discoloration or fading of inkjet recorded images. Many of ink receiving layers provided on the surfaces of special paper for photo quality inkjet employ a material such as porous white inorganic substance and the like in order to dry ink sooner and to make bleeding less in high image quality, resulting in that on such recording paper, discoloration or fading caused by ozone gas is noticeably observed. As this phenomenon of discoloration or fading caused by ozone gas is a characteristic of inkjet images, improvement of ozone gas fastness is one of important challenges in the inkjet recording method.
In order to extend the use field of the printing method using ink in the future, ink compositions to be used for inkjet recording and colored articles colored therewith are strongly required to have light fastness, ozone gas fastness, moisture fastness and water fastness which are further improved.
While inks with various hues have been prepared from various dyes, black ink among them is an important ink to be used for both mono color and full color images. Many dyes for these black inks have been proposed up to the present, but any product sufficiently satisfying the market requirements has been not provided yet. Many of the proposed coloring matters are azo coloring matters, and among them, disazo coloring matters such as C.I. Food Black 2 have such problems that the optical density of images is low, water fastness and moisture fastness are too low, and light fastness and gas fastness are not sufficient. Polyazo coloring matters where the conjugate system is extended have such problems that bronzing phenomenon having metallic luster is apt to generate partially on recorded images because their water-solubility is generally low, and that light fastness and gas fastness are not sufficient. In addition, in the case of metal-containing azo coloring matter proposed in large numbers as well, some of them have good light fastness but also have such problems that they are not preferable in view of safety to creatures and environment due to containing a metal ion and that ozone gas fastness is extremely poor.
A compound (coloring matter) for black ink for inkjet which has been improved on ozone gas fastness which becomes the most important problem in recent years includes, for example, the compounds described in Patent Literatures 1 and 2. These compounds don't sufficiently satisfy the market requirements on ozone gas fastness and their light fastness is not sufficient, either. In addition, azo compounds having a benzimidazolopyridone skeleton which is a characteristic of the coloring matter compound for black ink of the present invention are described in Patent Literatures 3 to 6 and the like. Patent Literature 4 and 5 disclose trisazo compounds, which have a symmetrical structure where two benzimidazolopyridone skeletons are further bonded to the both ends of a linking group containing an azo structure by an azo structure, and any similar compound to the unsymmetrical trisazo compound of the present invention has not been disclosed yet. Further, there are only a small number of water-soluble compounds and there is no example of their use as a black compound for inkjet ink.
Black inks where yellow to orange dyes are further formulated in a black dye have been proposed, for example, in Patent Literatures 7 to 11, but any product has not been provided yet which sufficiently satisfies the market requirements in terms of print quality, ozone fastness and light fastness.
Patent Literature 12 discloses the compound represented by the following formula (5) which is one of the coloring matter compounds to be used in the present invention.
In addition, a black ink containing two black dyes and yellow to orange dyes is proposed as an ink composition improved on ozone fastness, light fastness and the like, in Patent Literature 13, and it has an excellent hue and color density as black color but has not sufficiently satisfied the recent market requirements yet in terms of the fastnesses (ozone fastness and light fastness).    [Patent Literature 1] JP 2003-183545    [Patent Literature 2] JP 2003-201412    [Patent Literature 3] JP 2006-509068    [Patent Literature 4] DE 2004488    [Patent Literature 5] DE 2023295    [Patent Literature 6] JP H05-134435    [Patent Literature 7] JP H7-122044    [Patent Literature 8] JP 3178200    [Patent Literature 9] JP H9-255906    [Patent Literature 10] JP 2003-286421    [Patent Literature 11] JP 2003-286422    [Patent Literature 12] WO 2006/001274 International Publication Pamphlet    [Patent Literature 13] JP 2005-68416